The present invention relates generally to an improved solder composition. More specifically, the present invention relates to an improved solder composition that contains no lead or bismuth yet still achieves superior soldering characteristics.
In the electronic manufacturing of printed circuit boards and the assembly of components thereon, it has been well known to employ solders containing tin and lead to provide mechanical and electrical connections. Solders which contain tin and lead typically yield highly reliable connections in both automated and manual soldering and provide a surface on printed circuit boards extremely conducive to soldering.
However, the use of such tin-lead solders in the manufacture of printed circuit boards and is assembly of components is becoming more and more problematic due to the toxic effects of lead exposure to workers and the inevitable generation of hazardous waste. Due to these environmental concerns, action is being taken to limit the amount of lead entering into the environment. Recently, federal and many state government agencies have begun to urge the electronics industry to find alternatives to tin-lead solders to reduce worker lead exposure and lessen the amount of lead waste going back into the environment. Long term exposure to lead can greatly effect the health of workers. For example, even small amounts of lead can affect the neurological development of fetuses in pregnant workers.
Tin-lead alloys of, for example, sixty (60%) percent tin, forty (40%) percent lead; and sixty-three (63%) percent tin, thirty-seven (37%) percent lead have historically been used for most electronic soldering operations. These alloys have been selected and are preferred because of their low melting temperatures, mechanical strength, low relative cost, as well as superior wetting characteristics and electrical conductivity.
Due to the materials used, many components and printed circuit boards are easily damaged by exposure to high temperatures during manufacture or assembly. Because of the heat transfer and distribution limitations and concerns, printed circuit boards are typically exposed to temperatures higher than the liquidus temperature of the alloy employed. In response to this concern, electronic manufacturers are exploring alternative alloys to replace the tin-lead alloys.
Compositions containing bismuth were attempted as a substitute for tin-lead alloys. However, these alloys were quickly rejected due to their many problems. For example, alloys containing bismuth exhibit poor soldering and extremely poor peel strength with reduced wetting characteristics for electronic soldering. Further, these bismuth alloys typically fail mechanical thermocycling strength tests that have been performed throughout the electronics industry. Since bismuth is typically mined from lead ores, lead production must be maintained to recover bismuth. Also, known reserves of bismuth are completely inadequate to serve the needs of the growing electronics industry. Therefore, alloys containing bismuth are unacceptable as a substitute for tin-lead alloys.
The prior art has not provided a solder composition exhibiting optimum wetting and flow properties without toxicity. Currently federal, military and commercial solder specifications lack a suitable non-toxic composition. The following prior art patents illustrate inadequate attempts to meet these needs.
Soviet Union Patent No. 183,037, issued to A. I. Gubin, et al. discloses an alloy containing antimony of 1.+-.0.3%; copper 2.+-.0.3%; silver 5.+-.0.3% and the remainder being tin and having a melting point of 225.degree.-250.degree. C. This alloy has a liquidus temperature that does not allow it to be used in electronic soldering because the soldering temperature required to flow the alloy would destroy the printed circuit board and many of the components. No feasible equipment or means currently exists to allow this alloy to be used for the purpose of electronic soldering or coating. Due to the high silver content, this alloy has an economic disadvantage in the marketplace.
U.S. Pat. No. 3,503,721, issued to Lupfer, discloses a tin-silver alloy of 96.5% tin and 3.5.+-.0.5% silver with wetting and electrical conductivity characteristics marginally acceptable to suit the needs of the electronics industry. However, this alloy has mechanical strength weaknesses that would prohibit its use on a wide range of electronic printed circuit board assemblies. For example, creep strength, a measure of flow under pressure, and percent elongation, metal stretching before fracture, are considerably lower than the tin-lead alloys now used. Even with the common tin-lead alloys, solder joints stress fractures are the cause of many field failures in printed circuit boards where vibration or temperature variations occur. In addition, the liquidus temperature of 221.degree. C. requires that automated soldering be accomplished at a temperature that in many situations would damage the printed circuit board and/or the components. Due to the high content of silver, the cost of this alloy is considerably higher than tin-lead alloys. For each percentage point of silver added to the alloy, the price increases by approximately $0.75/lbs. (based on a silver market of $5.00/troy ounce.)
U.S. Pat. No. 4,778,733, issued to Lubrano, et al., discloses an alloy containing, by weight, 0.7% to 6% copper; 0.05% to 3% silver; with the remainder being tin with a temperature range of 440.degree.-630.degree. F. This alloy does not use antimony in its composition. As a result, this alloy has a melting temperature that is too high to be used in a wide range of electronic soldering applications without damaging printed circuit boards or components. In addition, the alloy disclosed by Lubrano, et al. exhibits inferior soldering performance, slow wetting times and mechanical strengths ill-suited to electronic assembly applications.
U.S. Pat. No. 4,695,428, issued to Ballentine, et al., discloses an alloy containing 0.5-4% antimony; 0.5-4% zinc; 0.1-3% silver; 0.1-2% copper; 88-98.8% tin. The zinc content in this alloy causes the alloy to oxidize quickly. This inhibits wetting and flow producing high dross formation which results in extremely high defect levels. The productivity lost in using such as composition for mass electronic soldering makes it an unacceptable alternative to tin-lead solders.
U.S. Pat. No. 4,758,407, issued to Ballentine, et al., discloses an alloy containing tin, copper, nickel, silver and antimony. All of the alloy combinations disclosed by Ballentine, et al. have liquidus temperatures in excess of those required for electronic assembly. The lowest disclosed liquidus temperature is 238.degree. C. which is unacceptable for use in the electronics industry.
Since heretofore no acceptable substitute for tin-lead alloys have been found, there is a need in the electronics industry for an alloy composition without lead or bismuth which can achieve the physical characteristics and application performance of tin-lead solder alloys but without the toxic elements.